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The present invention relates to compression release engine brakes, and more particularly, to an electromechanical latching rocker arm type of engine brake mechanism.
A compression release engine brake is a device for use with an internal combustion engine which operates by allowing compressed gas (typically, air) to be released by the exhaust valve during the compression stroke, near the top dead center position of the piston within the cylinder. As a result, energy is expended by the engine to compress the gas, but no useful work is returned to the piston, and the net result, with an engine brake device functioning on one or more cylinders, is an effective braking of the engine. Typically, the fuel supply to the engine (e.g., fuel injectors) is turned off during operation of the engine brake.
By braking or xe2x80x9cretardingxe2x80x9d the operation of the engine, the speed of the vehicle being propelled by the engine may be substantially reduced, thereby reducing the need to use the conventional wheel brakes of the vehicle. Thus, the use of an engine brake will substantially increase the life of the conventional wheel brakes, and will also provide for safer operation of the vehicle, especially when operating in hilly terrain. In other words, even if there is a problem with the conventional wheel brakes, when actuated by the vehicle operator, the engine braking system will still provide enough braking capacity to bring the vehicle safely under control.
Although engine brakes are used primarily on larger vehicles such as trucks (and typically, on engines having a displacement of about 10 liters or more) and most trucks are equipped with diesel engines, it should be understood that the engine brake of the present invention could be applied to either a diesel or Otto cycle type of engine. Furthermore, although the present invention will be described in connection with a center-pivot rocker arm type of valve gear train, those skilled in the art will understand that the invention may be used advantageously with any pivoting rocker arm type of valve gear train, for reasons which will become apparent subsequently.
Conventional compression release engine brakes typically include hydraulic circuits for transmitting a mechanical input to the exhaust valves to be opened, as part of the braking event. Such hydraulic circuits typically include a master piston which is reciprocated in a master piston bore by a mechanical input from the engine, such as the pivoting movement of the fuel injector rocker arm. Hydraulic fluid in the circuit transmits the motion of the master piston to a slave piston in the circuit which, in turn, reciprocates in a slave piston bore in response to the flow of hydraulic fluid in the circuit. The slave piston acts either directly or indirectly on the exhaust valve to be opened to achieve the engine braking.
One of the problems associated with the conventional prior art compression release engine brake system of the hydraulic type is that the source of hydraulic pressure (such as the master piston described above) would typically operate continuously, thus wasting engine horsepower when the vehicle is operating in an environment in which the engine brake is seldom used, for example, when travelling over relatively flat roads. Also, in looking toward the future, it is anticipated that most fuel injection systems for truck diesel engines will be of the xe2x80x9ccommon railxe2x80x9d type, in which fuel is communicated through a common passage, rather than having individual fuel injectors. Elimination of the fuel injectors, and the associated injector rocker arms, would eliminate what is effectively a xe2x80x9cfreexe2x80x9d mechanical input to the hydraulic pump.
The typical compression release engine brake sold commercially by Jacobs Vehicle Systems is one which uses the pivoting motion of the fuel injector rocker arm as the mechanical input to the pump to supply hydraulic pressure to an engine braking mechanism. In the systems currently supplied by Jacobs (under the trademark xe2x80x9cJake brakexe2x80x9d), the exhaust valve is subjected to undesirable valve motion, in both the opening and closing directions of movement. The result is that the engine braking system xe2x80x9cdistressesxe2x80x9d the exhaust valve, thus decreasing the effective life of those exhaust valves which are part of the engine braking system, and increasing the maintenance costs for the engine. In addition, in many vehicle engine applications, the size and weight of the conventional Jake brake is such that other parts of the engine, such as the rocker cover, must be modified to accommodate the engine braking system. Thus, the overall cost of using the prior art system is likely to be excessive, and may limit the commercial application of the prior art engine brakes.
It would be desirable to have an engine braking system which does not require a source of hydraulic pressure, for the reasons mentioned above. However, it would also be important, if the system were mechanical, and especially if the system involved some sort of xe2x80x9clost motionxe2x80x9d device, for the transition between the unactuated and actuated conditions to occur in less time than it takes for the cam shaft to make one complete revolution. In fact, it would be quite desirable for the system to be able to make the required transition in less than half of the cycle of the cam shaft, i.e., between the normal exhaust valve event and the time of the braking event, which occurs at xe2x80x9cTop Dead Centerxe2x80x9d of the compression stroke.
Accordingly, it is an object of the present invention to provide an improved compression release engine brake mechanism which does not require hydraulic actuation, and therefore, avoids the complexity and expense associated with fluid pressure operated devices, as well as the sealing problems associated therewith, and the wasted engine horsepower to maintain such a system pressurized.
It is another object of the present invention to provide an improved engine braking mechanism which accomplishes the above-stated object without adversely affecting the exhaust valve in terms of additional loading on the valve and the resulting reduction in the useful life of the valve.
It is a more specific object of the present invention to provide an improved engine braking mechanism which does not involve any modification of the normal exhaust event for the exhaust valve, but instead, merely adds the braking event to the cam profile.
It is a still further object of the present invention to provide an improved engine braking mechanism in which movement of the mechanism into the xe2x80x9cbrakingxe2x80x9d mode is triggered by the release of a stored energy spring for fast actuation.
The above and other objects are accomplished by the provision of an improved compression release engine brake assembly adapted for use with an internal combustion engine of the type including an engine piston reciprocally mounted within a cylinder for cyclical successive compression and expansion strokes. An exhaust valve is operable to open in a normal exhaust lift event and in a brake lift event. The engine includes an exhaust valve actuating mechanism for imparting reciprocable movement to the exhaust valve in response to rotation of a cam shaft including a cam profile defining a base circle portion, a normal lift portion and a brake lift portion rotationally displaced from each other on the cam profile. The exhaust valve actuating mechanism includes a cam follower adapted for operative engagement with the cam profile and a valve engagement portion adapted for engagement with the exhaust valve.
The improved engine braking assembly is characterized by the exhaust valve actuating mechanism including a lost motion device disposed in series relationship with the exhaust valve and being moveable between a normal, lost motion condition and an actuated condition not providing lost motion, in response to movement of an input member between first and second positions, respectively. A biasing spring normally biases the input member toward the first position. An energy storage spring is operable, after being compressed to an energy storage condition, to be able to bias the input member toward the second position in opposition to the force of the biasing spring. A latch mechanism is operable to displace the energy storage spring from a non-compressed condition to a compressed condition in response to the movement of the valve actuating mechanism as the cam follower traverses the normal lift portion of the cam profile. The latch mechanism is operable to release the energy storage spring just before the cam follower traverses the brake lift portion of the cam profile, thereby permitting the energy storage spring to displace from the compressed condition to a relatively non-compressed condition, and thus move the input member to the second position, moving said lost motion device to said actuated condition.